Vapor precipitation of polymers from solvent polymer blends by azeotropic spray drying

ABSTRACT

A process for removing contaminating solvents from solutions containing solid solutes comprising contacting a co-solvent that forms an azeotrope with the contaminating solvent forming a mixture thereby and spray drying the contaminating solvent co-solvent mixture in the presence of the co-solvent at a temperature equal to or above the temperature where the azeotrope distills whereby the contaminating solvent has been azeotropically distilled away from the solute. The process is particularly useful for polymeric solutes, especially silicones.

This is a divisional of application Ser. No. 08/552,685 filed on Nov.03, 1995, now U.S. Pat. No. 5,618,902.

FIELD OF THE INVENTION

The present invention relates to a process for removing undesired, i.e.contaminating, solvents from solvent solute compositions by means of ahigh temperature vapor wherein the vapor forms an azeotrope with thecontaminating solvent(s) present in the solution wherein when thesolution is so processed the azeotrope is distilled and the solute ispurified of the contaminating solvent.

More particularly the present invention relates to the spray drying inthe presence of a solvent vapor or azeotropic mixture of solvent vaporsof an organic solvent dispersion of a polymeric solute to form afree-flowing powder free of contaminating solvent. A preferred polymeris a silicone polymer.

BACKGROUND OF THE INVENTION

A method for making silicone powders is shown in U.S. Pat. No. 4,935,484('484). Aqueous colloidal suspensions of silicone polymer, copolymer arespray dried to produce silicone powders having an average particlediameter of 10 to 150 nm. Although the powders so-produced may beutilized in a variety of applications, the powder still contains theemulsifying agent used in its preparation as an impurity.

As taught in the '484 patent, the silicone powder made by the process ofspray drying an aqueous silicone polymer, copolymer can be used inplastics as a substitute for finely divided silica that has beenrendered hydrophobic. However, the aqueous spray dried powder producedby the process of the '484 patent is incompatible with other siliconefluids when the polymer, copolymer so spray dried is a polymer,copolymer of triorganosiloxy units combined with tetrasiloxy units.

Because of the deficiencies of the '484 patent, it is thereforedesirable to be able to produce free-flowing organosilicon powdershaving a particle size in the range of about 0.10 to about 200 micronsand an aggregate size in the range of 10 microns to about 200 micronswhere such a powder would be compatible with other silicone polymers andtherefore combinable. Such materials where this would present a usefulfeature are in combinations with alkenyl substituted organopolysiloxanesand hydride silicone fluids.

SUMMARY OF THE INVENTION

Broadly conceived the present invention utilizes the formation ofazeotropes in conjunction with the process of spray drying to removeunwanted solvents from solutions and thereby purify the solution of theunwanted solvent or remove the solvent entirely to produce a solid thatis substantially free of the unwanted solvent.

Thus there is broadly provided a process for removing a contaminatingsolvent from a solution containing one or more solutes comprising:

(a) contacting said solution with a co-solvent having a boilingtemperature wherein said co-solvent is immiscible with said solute andwherein said co-solvent boils at a temperature different from theboiling temperature of said contaminating solvent and wherein saidco-solvent forms an azeotrope with said contaminating solvent; and

(b) spray drying said solution contaminated with said solvent in thepresence of said co-solvent wherein said solvent and said co-solventdistill azeotropically,

whereby said contaminating solvent is removed from said solute.

As particularly embodied, the process of the present invention may beapplied to solutions containing polymers, more particularly to polymersof silicones, and most particularly to silicone polymers having theformula:

    M.sub.i D.sub.j T.sub.k Q.sub.p

where the subscripts j and k are zero and the subscripts i and p arenon-zero and positive. When the process of the present invention isapplied to the preferred silicone polymer, a finely divided powderedsilicone polymer results that is substantially free of the contaminatingsolvent (or contaminating solvent mixture).

DETAILED DESCRIPTION OF THE INVENTION

Polymers resulting from the solution synthesis or solution mediatedsynthesis or similar processes can frequently be an intractable solid ifthe organic solvent is removed. When removal of the solvent after thepolymer or copolymer is synthesized would produce an intractable solid,the solvent generally is not removed and the product is used as adispersion or solution in the solvent of its manufacture. This hasnecessitated the development of high solids content synthetic approachesor novel methods of removing the contaminating solvent.

Polymers, or for the purposes of the process of the instant inventionoligomeric materials synthesized in an organic solvent or prepared by asolvent assisted or mediated process, are thermoplastic polymersincluding but not limited to polyethers, polysulfides, polyimides,polyamides, polyetherimides, polyetheramides, polysulfones,polyurethanes, polyesters, polycarbonates, silicones, alloys of any ofthe foregoing and mixtures thereof. The process of the present inventionis particularly well-suited for polyesters, polycarbonates, siliconesand the like. In the case of silicones there is a wide variety ofcompositions that constitute the silicone polymers that may be dried bythe process of the present invention:

MDM,

TD,

MT,

MDT,

MDTQ,

MQ,

MDQ,

MTQ and the like. These formulas are individual selections from ageneral formula for silicones comprising:

    M.sub.i D.sub.j T.sub.k Q.sub.p

where the subscripts i, j, k, and p may independently be zero or apositive number subject to the proviso that when p is non-zero one of i,j, and k must also be non-zero. For purposes of clarity, M is amono-functional group having a generic formula R₃ SiO_(1/2), D is adifunctional group having a generic formula R₂ SiO_(2/2), T is atrifunctional group having a generic formula RSiO_(3/2), and Q is aquadrifunctional group having the formula SiO_(4/2). In these examples,the substituent R groups may be any of a very large variety of organicradicals, which of course may be independently selected. In the case ofthe M groups all three R groups may be different, two may be alike, orall three may be identical. Similarly for the D groups, the R groups maybe identical or different. In building up silicone polymers, copolymersmay be created where two or more different M, D, or T groups areutilized in the synthesis. Thus the above list may be permutedindefinitely. Usually these monovalent R substituent groups are selectedfrom hydrogen, one to forty carbon alkyl groups, two to forty carbonalkenyl groups, six to forty carbon aryl groups, and seven to fortycarbon alkylaryl (or alkaryl) groups. Further these substituent Rgroups, when they are not hydrogen, may be partially halogenated withfluorine, chlorine, bromine, and iodine or substituted with oxygen orsulfur derivatives such as hydroxyl or mercapto groups, esters,thioesters, ethers, thioethers and the like, or any combination ofthese.

Some of the solvents used for these polymerization and oligomerizationprocesses are benzene, toluene, xylene, mixed aromatic solvents,paraffinic hydrocarbons, mixtures of 5 to 20 carbon atom paraffinicsolvents, and one to ten carbon atom alcohols. Recently this problem hasbeen overcome by removing the solvent through spray drying as taught inU.S. Pat. Nos. 5,319,040; 5,324,806; and 5,357,007. This produces afinely divided polymer powder that is not intractable, that is easy tohandle and that disperses well in other silicones. The technique isgeneralizable to other solvent contaminated polymer compositions. Forexample, polyesters, polycarbonates and the like which are synthesizedvia a solvent mediated polymerization reaction can form intractablesolid masses above certain threshold molecular weights if the solvent isremoved. Thus these recently developed techniques of spray drying mayprovide a convenient means of producing higher molecular weight polymersand conveniently removing most of the solvent from the product producinga finely divided easily handled powder.

While the techniques disclosed in the '040, '806, and '007 patentsremove a significant fraction of the solvent by the simple expedient ofspray drying, not all of the solvent is removed. This is either abenefit or a detriment depending on whether the presence of traceamounts of solvent assist converting the product to useful forms orwhether the solvent represents a contaminant that must be removed. Inthe case of trace aromatic solvents such as benzene or toluene, if theproduct will be used in applications which do not involve the humanbody, the solvent contamination by aromatic solvents may be of noparticular concern. In contrast, for cosmetic and personal careformulations or where the resin or thermoplastic will be a component ofa medical device such solvent contamination assumes more seriousproportions.

The present invention provides a significant departure from these priorteachings in that the spray drying of the solvent polymer mixture isconducted in the presence of an additional solvent or solvent mixture,hereinafter referred to as a co-solvent By choosing the co-solvent, sothat the co-solvent forms an azeotrope with the solvent or solventmixture carrying the polymer, copolymer, resin or thermoplastic, theprocess of spray drying also simultaneously acts as an azeotropicdistillation and the polymer, copolymer resin or thermoplasticprecipitates as a finely divided powder in the spray drying apparatus.This new process makes it possible to reduce solvent contamination ofthe polymer, copolymer, resin or thermoplastic to significantly lowerlevels than can be achieved by the process of spray drying unassisted bya co-solvent.

The organic solvents which mediate the synthesis or which are used assolvents for the synthesis of polymers, copolymers, resins, orthermoplastics are typically solvents that have a boiling point below250° C. Thus for example if a polymeric material is prepared by asolvent mediated polymerization where the solvent is benzene, the spraydrying of the resulting benzene (solvent) contaminated polymer isaccomplished using water as a co-solvent forming the classicbenzene-water azeotrope. Performing the azeotropic spray drying attemperatures equal to or above where the benzene water azeotropedistills results in the azeotropic distillation of the azeotrope awayfrom the polymer resulting in the precipitation of the polymer. Suitableco-solvents are water, ethylene glycol, propylene glycol, butyl alcohol,isobutyl alcohol, allyl alcohol, acetic acid, 2-chloro-ethanol, ethylenediamine, nitroethane, pyruvic acid, 1-chloro-2-propanone,epichlorohydrin, propionic acid, and 1,2-propanone diol.

In order to remove the contaminating solvent, the choice of co-solventshould be one that is not miscible or soluble with the solute orpolymer. The use of an immiscible or insoluble co-solvent duringazeotropic spray drying results in a polymer (or solute) having a lowconcentration of the contaminating solvent without introducing asecondary contamination by the co-solvent A further consideration, isthat the co-solvent selected for azeotropic spray drying must form alower boiling point azeotrope with the contaminating solvent relative tothe boiling point of the co-solvent itself. This requirement leads to amulti-phase system where there is at least one vapor phase and tworequired liquid phases. The process of the present invention requiresthat the co-solvent component of the azeotropic mixtures be selected sothat the contaminating solvent distills from the lower boiling liquidphase present during spray drying. The existence of these phases,partitions the contaminating solvent into a lower boiling liquid phasewhich is rejected during the spray drying process by azeotropicdistillation thereby purifying the solute or polymer.

Spray drying of the solvent contaminated polymer in the presence of aco-solvent may be accomplished by techniques known in the art or with aspray dryer having at least one vortex atomizer nozzle, a spray dryerhaving a two fluid nozzle, or as disclosed in U.S. Pat. No. 5,248,087 aspray dryer having a droplet generator.

The temperatures of the spray drying process can vary from as low as 50°to as high as 300° C., depending on the solvent of preparation and thechoice of co-solvent. Preferably these ranges are 90° to 300° C., arange of 100° to 250° C. is more preferable, and a range of 150° to 200°C. is most preferred. In the event that it is desired to lower the heatduty on the spray dryer these preferred ranges change to 60° to 250° C.preferred, 65° to 200° C. more preferred, and 70° to 190° C. mostpreferred. Further when high boiling solvents and co-solvents are usedthese preferred ranges again change to 100° to 300° C., 150° to 300° C.more preferred, and 200° to 300° C. most preferred.

There is provided by the present invention, a method for making afree-flowing silicone powder. Such free-flowing powder may be moreeasily blended with other silicone materials resulting in more stableformulations. One specific embodiment of the method of the presentinvention comprises:

(1) forming an organosiloxane hydrolyzate, considered a polymer forpurposes of the process of the present invention, comprisingtriorganosiloxy units and tetrasiloxy units having an triorganosiloxy totetrasiloxy ratio of about 0.65:1 to about 3:1,

(2) separating the organosiloxy hydrolyzate from the mixture of step (1)in the form of an organic solvent dispersion or solution, and

(3) spray drying the organic solvent dispersion or solution resultingfrom step (2) in the presence of steam, or an azeotropic mixture ofsteam and the solvent or solvent mixture used in step (2).

The triorganosiloxy groups, M, have the formula:

    M=R.sub.3 SiO.sub.1/2,

where the R groups, which may be independently selected, are hydrogenand one to forty carbon atom monovalent hydrocarbon radicals. R ispreferably selected from the group consisting of hydrogen, one to sixcarbon atom alkyl groups, six to twenty carbon atom aryl groups, andseven to twenty carbon atom alkylaryl groups, R is more preferably amethyl group. Since R is defined in terms of a carbon framework only,there may be other substituents functionalizing the R substituents. ThusR includes halogenated methyl groups such as trifluoromethyl,trichloromethyl, halogenated higher alkyls such as trifluoropropyl,halogenated aryls such as chlorophenyl, oxygenated aryls such asphenolic substituents and the like. Additonal triorganosiloxy units, M'and M", may comprise part of the mixture being hydrolyzed. Suchtriorganosiloxy groups have the following formulas:

    M'=R.sub.2 R.sup.1 SiO.sub.1/2 (alternatively R.sub.2 R.sup.1 Si), and

    M"=R.sub.2 HSiO.sub.1/2 (alternatively R.sub.2 HSi)

where R is as previously defined and R¹ is a two to forty carbon atomalkenyl group. The triorganosiloxy units may additionally comprise Dunits of the formula:

    R.sup.2 RSiO.sub.2/2 (alternatively R.sup.2 RSiO) and

T units of the formula

    RSiO.sub.3/2 (alternatively RSiO.sub.3)

where R is as previously defined and R² is selected from the groupconsisting of R, R¹ and H. The quantities of D and T units that may bepresent independently range up to about 25 mole percent

The tetrasiloxy units have the formula:

    SiO.sub.4/2

The organic hydrolyzate included within the description of the processof the present invention may be dispersible or soluble in aromaticsolvents such as benzene, toluene, xylenes, xylol and the like.

The organosiloxane hydrolyzate may contain from about 0.02 to about 5weight percent and preferably from about 0.1 to about 3 weight percentand most preferably from about 0.5 to about 2.5 weight percent hydroxyradicals based on the total weight of organosiloxane.

A method for producing the triorganosiloxane hydrolyzate is disclosed inU.S. Pat. No. 2,676,182 ('182). In the '182 patent a silica hydrosol isreacted under acidic conditions with a source of triorganosiloxy unitssuch as hexamethyldisiloxane or a hydrolyzable triorganosilane such as atrimethylchlorosilane followed by recovering a silicone polymer orcopolymer that is dispersible in an organic solvent such as benzene.

A specific embodiment of the instant invention applicable primarily tosilicones having the general structural formula of MQ, MM^(vi) Q,MD^(vi) Q, MM^(H) Q, MD^(H) Q, MDTQ TD and permutations thereofcomprises:

(1) Preheating an organic solution of the silicone to a temperatureranging between 50° and 300° C.

(2) Pressurizing the solution and passing the solution through a twophase spray nozzle wherein the solution contacts the vapor of aco-solvent in which the silicone is sparingly soluble and which forms anazeotrope with the organic solvent. Further, the organic solvent shouldbe present in a molar quantity that is greater than that of theco-solvent or co-solvent mixture for purposes of forming an azeotropethat will function in the process of the present invention. The effectof these conditions is to precipitate particles of silicone from thesolvent at the same time that the solvent is stripped from the siliconeby the action of the co-solvent in forming an azeotrope. Any remainingliquid, accompanying the silicone particles will usually be theco-solvent

The ability of these solvent systems to remove virtually all of thesolvent while leaving the solids wetted with co-solvent adds energyefficiency. The solids wetted with co-solvent can then be dried in anindirect contact heat exchanger that allows for heat transfer withoutincreasing the mass of the process stream. An indirect heat exchanger ismore energy efficient by comparison to providing all of the heat inputfor the process from a heated co-solvent.

Particles sizes produced by the process of the instant invention rangefrom about 0.40 to about 1,000 microns, preferably 10 to 200 microns,and most preferably 20 to 100 microns.

(3) Heating the vapor/liquid/solids stream to vaporize the remainingco-solvent liquid and superheat the vapor.

(4) Separating the solids from the superheated vapor via a cycloneseparator, fritted metal filters, woven or spun bag filters, orcartridge filters. Cyclone separators may be preferred because of theirability to be designed to capture fine particulate matter moreefficiently.

(5) Collection of the recovered solids in a containment vessel suppliedwith a vapor purge, preferably air or nitrogen. The vapor purgedisplaces entrained co-solvent vapor from the free void volume of thesolids, reducing thereby the final solvent content of the solids.

(6) Condensing the vapor stream with a direct contact or indirectcontact heat exchanger. When, as preferred the solvent and co-solventare immiscible, a direct contact condenser followed by a phase separatorprovides a particularly efficient means of recycling solvents.

(7) Providing a low pressure outlet for non-condensable gases so thatthe system runs at less than atmospheric pressure, between 2 and 760 mmHg, preferably between 25 and 700 mm Hg, more preferably between 50 and300 mm Hg, and most preferably between 100 and 250 mm Hg. By using areduced pressure the partial pressures of all the components arereduced. This is particularly important in reducing the amount ofentrained co-solvent in the free void volume of the precipitated solids.One particularly useful arrangement for moderately low pressureapplications is the use of a liquid jet eductor that provides the directcontact condenser and as a consequence the energy necessary to produceor maintain a low pressure upstream of the eductor. The eductordischarge can then be decanted when immiscible solvent and co-solvent isused, a portion cooled below 27° C., followed by recycle to the eductoras the motive fluid.

Any silicone having a melting point greater than 25° C., preferablygreater than 35° C., more preferably greater than 43° C., and mostpreferably greater than 50° C. and soluble in an organic solvent may bestripped of solvent by the process of the instant invention subject tothe limitation that there exists a co-solvent with which the silicone isimmiscible and with which the solvent and co-solvent form two liquidphases that will form a low boiling, solvent rich azeotrope.

While a specific embodiment of this solvent removal process involvesspray drying to form a free-flowing solid, the process of the presentinvention might also be employed to dry materials that are normally hardto purify viscous liquids. If this process embodiment is employed, aliquid would collect in the bottom of the spray drier rather than asolid free-flowing powder. The choice of co-solvent in the case of aliquid drying process would then be governed by the additionalconsideration of immiscibility with the liquid phase being dried as wellas well as forming an azeotrope.

The process of the present invention is not limited to solutions ofpolymers but rather may be extended to include the removal of unwantedsolvent from solutions containing desirable products or solutes byadding thereto a co-solvent or a mixture of co-solvents that forms anazeotrope with the unwanted or contaminating solvent and spray dryingthe solution comprising solute, contaminating solvent and co-solventthereby removing the contaminating solvent.

A particularly important beneficial aspect of the present invention isthat it may be utilized to purify silicone polymers of objectionable(and therefore contaminating) aromatic solvents. This is importantbecause the most convenient synthetic methods used to produce siliconepolymers frequently utilize aromatic solvents. One notable example is inthe production of silicones possessing long chain hydrocarbylsubstituents either as on chain or endstopped substituents. This is aproblem because a significant application of various silicones is inpersonal care products where trace levels of aromatic solvents areundesirable from the standpoint of health and safety and thus levels ofthose types of contaminating solvents in personal care consumer productsare regulated.

All United States patents referenced herein are specifically herewithand hereby incorporated by reference.

EXPERIMENTAL EXAMPLE 1

A 60 weight percent toluene solution of an MQ resin, having an M/Q molarratio of about 0.65, a number average molecular weight of about 3,000grams per mole and a silanol content of about 2.0 weight percent wasvapor precipitated with water vapor at atmospheric pressure into a bagfilter separator to capture the finely divided solids so produced. Thevapor phase was condensed in a shell and tube type condenser. The solidsas recovered contained 14 weight percent moisture, as water. Theparticle size distribution ranged from 0.40 microns to about 100 micronswith a mean particle size of 14 microns. The solid particulate resinrecovered from the bag filters was dried to remove the residual moisturecontent. Using a 20 centipoise polydimethylsiloxane fluid, it waspossible to disperse the particulate resin so prepared, producing anoptically transparent mixture using only mixing by hand. This indicatesthat the particulate silicone produced by the technique of the presentinvention is readily dispersible in low molecular weight siliconefluids.

A 3.518 g sample of the wet particulate silicone recovered from the bagfilter was heated to 150° C. in a sealed septum vial and a 3 mL gassample from the head space over the silicone was injected using a model19395A Hewlett-Packard head space analyzer onto a 105 meter RTX-1capillary gas chromatographic column. By comparison against a referencemixture containing 0.0009 g toluene per 20 mL head space, no toluene wasdetected in the sample. The limit of detection was 7 ppm by weight.Therefore, the process of the instant invention produces siliconesamples with less than 7 ppm aromatic solvent in the product. Clearly,the process may be operated under more stringent conditions to achieveeven higher levels of contaminating solvent removal. Thus it ispreferable to remove the contaminating solvent to levels below 3 ppm byweight, more preferably below 1 ppm by weight, and most preferably below0.5 ppm by weight.

EXAMPLE 2

For purposes of comparison, the same MQ resin toluene solution as inExample 1 was spray dried using a Niro portable HT spray dryer equippedwith a two fluid nozzle. Spray drying was achieved at inlet/outlettemperatures of 200°/110° C. respectively employing nitrogen gas as adrying agent at a rate of 85 kg/hr. The weight ratio of nitrogen dryinggas to MQ resin (as a 60 weight percent solution in toluene) was 0.72.The resulting MQ resin powder had an average particle size of 68 micronsand contained 1.5 weight percent residual toluene solvent.

This example demonstrates by comparison that the use of an azeotropeforming co-solvent in spray drying a solution of a silicone results in asignificant reduction in the contamination of the resulting powder bythe contaminating solvent

DEFINITIONS

The endstopping groups M can be alternatively defined as:

    M=R.sub.3 Si,                                              a)

in contrast to the usual definition where:

    M=R.sub.3 SiO.sub.1/2.                                     b)

Both of these definitions may sometimes create formal structuralproblems related to the presence or absence of a divalent oxygen linkingspecies, if they are used interchangeably without regard to theformalities of the stoichiometric conventions being employed. Whendefinition a) is used an additional oxygen atom must be inserted in theleft side of structural formulas such as MDM, i.e. MODM, assuming D hasbeen defined as: ##STR1## When definition b) is used the structuralformula MDM in order to be precisely correct must be written MD(SiR₂)Min order to avoid an extra oxygen atom in the formula. The generallyaccepted trivial (or idiomatic) notations in silicone chemistry assumethe practitioner having ordinary skill in the art (which for chemistry,according to the American Chemical Society, is possession of a Master ofScience degree) recognizes the need for adding or deleting a divalentoxygen atom as necessary when these structural abbreviations are used,particularly if they are used interchangeably without regard tostoichiometric subtleties, depending on which definitions are used sincethe typical abbreviation is MDM and not MODM or MD(SiR₂)M even thoughcontextually the latter two are correct depending on the definition of Mwhile the former can be idiomatically or trivially correct or incorrectdepending on whether definitions are mixed.

When fractional subscripts are used, conceptually the atom so designatedis shared equally between two centers. As generally used in siliconechemistry this is for an oxygen atom shared equally between two siliconatoms. Thus a subscript of 1/2 refers to one catenating oxygen in acatenated structure of alternating silicon and oxygen atoms, 2/2 refersto two catenating oxygens, 3/2 refers to three catenating oxygens, and4/2 refers to four catenating oxygens.

Likewise, a superscript H on one of the structural componentabbreviations of M, D, or T, e.g. M^(H) or D^(H), indicates thesubstitution of a hydrogen atom for one of the R substituents indicatinga conversion of the species to a so-called hydride or hydrogen siloxane.Further a superscript vi refers specifically to a vinyl substituent andgenerally to an alkenyl substituent; usually, alkenyl substituents areselected from the group of two to forty carbon atom alkenyl groups.

As used herein, Applicants define a contaminating solvent to be anyunwanted solvent present in the product.

Having described the invention that which is claimed is:
 1. A processfor removing a contaminating solvent having a boiling temperature from asolution containing a solute comprising:(a) contacting said solutionwith a co-solvent having a boiling temperature wherein said co-solventis immiscible with said solute and wherein said co-solvent boils at atemperature different from the boiling temperature of said contaminatingsolvent and wherein said co-solvent forms an azeotrope with saidcontaminating solvent; and (b) spray drying said solution contaminatedwith said solvent in the presence of said co-solvent wherein saidsolvent and said co-solvent distill azeotropically, whereby saidcontaminating solvent is removed from said solute.
 2. The process ofclaim 1 wherein said spray drying is conducted at a pressure no greaterthan atmospheric pressure.
 3. The process of claim 1 where the solventis selected from the group of benzene, toluene, xylene, paraffinicsolvents having from 1 to 20 carbon atoms, alcoholic solvents havingfrom 1 to 10 carbon atoms and mixtures thereof.
 4. The process of claim3 where the solvent is selected from the group consisting of benzene,toluene and xylene.
 5. The process of claim 4 where the co-solvent isselected from the group consisting of water, ethylene glycol, propyleneglycol, butyl alcohol, isobutyl alcohol, allyl alcohol, acetic acid,2-chloro-ethanol, ethylene diamine, nitroethane, pyruvic acid,1-chloro-2-propanone, epichlorohydrin, propionic acid, and 1,2-propanonediol.
 6. The process of claim 5 here the co-solvent is water.
 7. Theprocess of claim 2 where the solvent is selected from the group ofbenzene, toluene, xylene, paraffinic solvents having from 1 to 20 carbonatoms, alcoholic solvents having from 1 to 10 carbon atoms and mixturesthereof.
 8. The process of claim 7 where the solvent is selected fromthe group consisting of benzene, toluene and xylene.
 9. The process ofclaim 8 where the co-solvent is selected from the group consisting ofwater, ethylene glycol, propylene glycol, butyl alcohol, isobutylalcohol, allyl alcohol, acetic acid, 2-chloro-ethanol, ethylene diamine,nitroethane, pyruvic acid, 1chloro-2-propanone, epichlorohydrin,propionic acid, and 1,2-propanone diol.
 10. The process of claim 9 wherethe co-solvent is water.